1. Field of the Invention
The present invention relates to a differential receiver which provides for estimation and/or tracking of frequency offset, together with compensation for the frequency offset.
2. Description of the Related Art
Differential receivers are popular in a variety of circumstances, such as in wireless local area networks (WLAN, for example, IEEE 802.11) or personal area networks (for example, Bluetooth®). A typical architecture employs differential quadrature phase shift keying (DQPSK). These receivers are attractive because of their low cost coupled with good performance and acceptable data transmission rates in the presence of noise.
Such communication systems suffer, however, from significant performance loss introduced by frequency offset, since it is difficult to accurately extract the data payload from a received signal in the presence of frequency offset. Frequency offset is common, given the low tolerances of the receivers and transmitters, and is also common in the presence of fading channels. Therefore, estimation and/or tracking of frequency offset, together with compensation therefor, is important to sustain adequate performance.
Conventional known systems estimate frequency offset either by calculating auto-correlation between a known pilot and a received signal, or by evaluating the cross-correlation between two identical symbols, such as symbols that might be found in the preamble of a data transmission. A conventional arrangement is shown in FIG. 1.
FIG. 1 illustrates one example of a conventional differential PSK (phase shift keying) receiver in the presence of an additive white Gaussian noise (AWGN) channel. A radio frequency signal is received by antennal 11, with the radio frequency signal encoding a digital data payload. RF front end 12 and RF-to-BB (baseband) converter 14 down-convert the radio frequency signal to a baseband signal, and further extract an in-phase component (denoted as “i”) and a quadrature phase component (denoted as “q”) that are respectively sampled by a pair of A/D converters 15. The in-phase signal and the quadrature signal are respectively filtered by low-pass filters 16 which eliminate adjacent channel interference and thereafter provide the signals IBB(n) and QBB(n) to differential detector 17. Differential detector 17 applies differential detection to the filtered signals to obtain a correspondingly demodulated PSK signal which is potentially corrupted by frequency offsets. Compensator 19 applies a frequency offset compensation based on an output of frequency offset estimation and tracking block 20 (which is described below), in order to reduce or remove the frequency offset. Phase extractor 21 extracts phase from the compensated signal, demodulator 22 demodulates the output from phase extractor 21, and decoder and bit slicer 24 decodes the demodulated output and provides the digital data payload at 25.
Reverting to differential detector 17, the demodulated PSK signal is often modeled mathematically by the complex-valued signal of the following equation:y(n)=ydi(n)+j*ydq(n)=aejψ  (Equation 1)where y(n) is the nth symbol, ydi and ydq are the in-phase and quadrature phase demodulated PSK signals, respectively, j is the imaginary coordinate for the complex value, and a and P represent the amplitude and phase of the received signal, respectively. Based on this mathematical notation, frequency offset estimation and tracking block 20 provides an estimate of the frequency offset by implementing an auto-correlation on the received signal according to the following equation:
                              2          ⁢          πΔ          ⁢                                          ⁢          fT                =                              ψ            ^                    =                      angle            (                                          1                N                            ⁢                                                ∑                                      n                    =                    0                                                        N                    -                    1                                                  ⁢                                                                  ⁢                                                      y                    ⁡                                          (                      n                      )                                                        ⁢                                                            y                      *                                        ⁡                                          (                                              n                        -                        L                                            )                                                                                            )                                              (                  Equation          ⁢                                          ⁢          2                )            where Δf is the frequency offset, T is time, {circumflex over (ψ)} is the estimate of phase, y(n) is the demodulated PSK signal from differential detector 17 and y*(n) is the complex conjugate thereof, and N and L are the block length of one training block and the distance therebetween. The relation between N and L are shown in FIG. 2, which shows that a typical RF transmission includes a preamble that prefaces the data payload, wherein the preamble includes N training signals y(n) that repeat at block distances separated by L symbols. Thus,y(n)=y(n−L)  (Equation 3)for n=0, . . . , N−1, which means that the training signals need to be repeated in order to obtain the frequency offset estimate.
As shown above, in conventional receivers, the estimation and tracking of frequency offset is computationally expensive. Specifically, quite a few number of symbols N are needed to estimate the frequency offset. Furthermore, the correlation of Equation 2 requires many complex-valued multiplications and complex-valued additions, especially when the number of samples N is large. Thus, in terms of complexity, chip area and/or power consumption, the conventional technique for estimation and tracking of frequency offset has its disadvantages.
Moreover, the range over which frequency offset can be estimated is limited by the block distance L: As the block distance L increases, the estimation range decreases. Since a large number of symbols N are needed, the value of L tends to increase, and conventional systems tend to exhibit a limited estimation range for estimation of frequency offset.